Projectile, adapted to be given a rotation on firing, which makes the projectile spin-stabilized

ABSTRACT

A projectile adapted to be given a rotation on being fired which makes the projectile spin-stabilized. 
     With the object of facilitating the terminal guidance of the projectile or increasing the effect of an explosive charge with a hollow-charge effect carried by the projectile, the projectile is provided with stabilizing fins (6-9) which are extended at a desired point in the trajectory of the projectile and brake the rotation of the projectile. The projectile is so dimensioned that its center of pressure (C2) is situated behind the center of gravity (G) of the projectile in the extended position of the fins (FIG. 2) and so that the center of pressure (C1) lies in front of the center of gravity of the projectile in the retracted position of the fins (FIG. 1), so that, with braked rotation, the projectile changes over from being spin-stabilized to being fin-stabilized. 
     The fins (6-9), which can consist of so-called wrap-around fins, are held in the retracted position, in an embodiment shown, by covering plates (2-5) which are held in place by a so-called base bleed unit (1). A delay device is adapted to be separated from the projectile at the desired point in the trajectory, so that the covering plates (2-5) are removed and expose the fins (6-9), as a result of which these can be extended.

This application is a continuation of application Ser. No. 269,039,filed May 14, 1981, now abandoned.

TECHNICAL FIELD

The present invention relates to a projectile adapted to be given arotation on firing, which makes the projectile spin-stabilized. Theinvention can be used, in particular, as a terminally guided projectileand in general for projectiles which receive an explosive charge with ahollow-charge effect.

BACKGROUND ART

The development in the artillery field, both land and sea artillery, hasrendered possible projectiles with an increased range of fire, forexample by means of a so-called base bleed unit. The increased range offire is naturally desirable but it leads to increased absolutedispersion of the projectiles. This increased dispersion is veryunfavourable, all the more so as a change in the threat picture hasbecome noticeable towards a greater frequency of smaller and harderelementary targets where each elementary target has to be combatted. Inorder to reduce the dispersion of the projectiles, terminal correctionor terminal guidance of the projectiles has been proposed. This meansthat a projectile is fired in a ballistic trajectory in conventionalmanner but at the end of the trajectory a target-seeking member andguidance electronics are activated which can lead the projectile to ahit or near hit on the target. Compared with a radical exchange of tubeartillery for missiles, a system with terminally corrected projectilesis less complicated than a robot because continuous guidance is notused. Moreover, the projectile is more difficult to intercept when itfollows a ballistic trajectory for great or greater portion of theflight.

Different solutions to this problem have been introduced. Conventionalartillery ammunition is spin-stabilized over the whole trajectory, thatis to say it has a high speed of rotation (of the order of magnitude of300-2000 rad/sec). Solutions to the problems of terminal guidance ofprojectiles which are spin-stabilized over the whole trajectory havebeen put forward. The advantages of such a system are that a completelyconventional firing can be effected with ammunition effects which differlittle in size and weight from conventional ammunition. Thedisadvantages are the very complicated guiding and the limited range ofcontrol as well as the very uncertain possibilities of realisation.

The target seeker is complicated and considerable difficulties arise incorrecting the course since the roll position of the projectile must bedetermined when the guiding signal is given. It has been proposed thatthe roll direction should be determined in relation to a referencedirection by means of the so-called rate-gyro and integration. Thisproposal is not without problems, however, because the gyro is sensitiveto acceleration and can drift. With projectiles which are fired with agun barrel, the sensitivity to acceleration is a particularly seriousproblem.

Thus a projectile which is spin-stabilized is altogether unsuitable foruse as a terminally guided projectile or in general if the projectile isto receive for example an explosive charge with a hollow-charge effectwhere the explosive radiation is adversely affected if the explosivecharge rotates.

An attempt to eliminate the disadvantages of a projectile which isspin-stabilized by discharging a useful load from the projectile isdisclosed in the Swedish Patent Specification No. 363 892. There aprojectile which is spin-stabilized is disclosed which is provided withbrake flaps which, at the desired moment in the trajectory, are loweredand brake the rotation of the projectile so that the projectile becomesunstable, after which the useful load of the projectile is thrown away.Since such a projectile thus becomes unstable as a result of the brakingof the rotation, it cannot serve as a terminally guided projectile or beprovided with an explosive charge with a hollow-charge effect since thatwould require that the projectile should be aerodynamically stable.

The majority of solutions hitherto put forward for the problem ofterminal guidance mean that the projectile is provided with so-calledrotating driving bands which means that the projectile has a low speedof rotation (of the order of magnitude 0-200 rad/sec) when it leaves themuzzle. This means that stabilizing fins must be extended immediatelyoutside the muzzle. The advantages of this system with low or no speedof rotation in the trajectory is that target seeking and guiding can befairly simple. Certain warheads, such as explosive charges with ahollow-charge effect, require a low speed of rotation to give a goodresult, as mentioned above. The disadvantages of this system are thatthe range of firing is adversely affected. Moreover, the dispersioneasily increases since the projectile is sensitive to disturbances atthe beginning of the trajectory, that is to say when the fins areextended, and the extension of the fins easily introduces disturbances.Moreover, with the solutions hitherto proposed, the length of theprojectile has greatly exceeded that which applies to conventionalprojectiles, which imposes new demands on the handling of ammunitionparticularly where automatic loading systems are concerned.

DISCLOSURE OF INVENTION

The present invention unites the advantages of the above system while atthe same time the disadvantages are reduced to a minimum as a result ofthe fact that the projectile according to the invention isspin-stabilized when it is fired, after which, at a desired point in thetrajectory of the projectile, the rotation is braked so that in thelatter part of the projectile's trajectory, the projectile isfin-stabilized. This is achieved as a result of the fact that theprojectile according to the invention is provided with stabilizing finsof the type which are known per se in fin-stabilized projectiles,wherein the fins are able to be extended from a retracted position onfiring to an extended position at a desired point in the trajectory ofthe projectile and so to brake the rotation of the projectile, andwherein the projectile is so dimensioned that its aerodynamic centre ofpressure in said extended position is situated behind the centre ofgravity of the projectile, so that during braked rotation, theprojectile changes over from being spin-stabilized to beingfin-stabilized.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described in more detail below with reference tothe accompanying drawings which show a preferred form of embodiment of aprojectile according to the invention, in which

in FIG. 1, the projectile, which is provided with a base bleed unit, isshown in the introductory phase of its trajectory,

in FIG. 2 the same projectile is shown when the base bleed unit has beendiscarded and the fins exposed and

in FIG. 3, the same projectile is shown when the optical system has beenexposed and the nose rudder extended.

In the figures, the same parts have been provided with the samereference numerals throughout.

BEST MODE OF CARRYING OUT THE INVENTION

The projectile is shown with a base bleed unit 1 which gives anincreased range of fire in known manner by giving off gas whichincreases the reduced pressure at the base end of the projectile. Theneed for end-phase correction increases, as stated earlier, withincreased range of fire. It will be seen, however, that the presentinvention is suitable, to an equally high degree, for all other types ofterminally corrected projectiles or for projectiles which carry anexplosive charge with a hollow-charge effect.

In its back portion, the projectile is provided with four stabilizingfins 6-9 of the type which is known per se in fin-stabilizedprojectiles. In the embodiment shown they consist of so-calledwrap-around fins, that is to say fins which, in the retracted position,largely follow the outer shell surface of the projectile. The fins 6-9are held in the retracted position by means of locking members in theform of four conventional covering plates 2-5, which are held in placeby the base bleed unit 1 as a result of the fact that this tightlysurrounds the back portion of the covering plates.

A delay device, not shown, in the projectile is dimensioned so as toinitiate, at a desired point of the trajectory, the throwing off of thebase bleed unit 1 so that the covering plates 2-5 are automaticallyremoved and expose the fins 6-9. These are so dimensioned, in knownmanner, that they extend through a combination of centrifugal forces andaerodynamic forces and afterwards, likewise in known manner, are lockedin the extended position.

It will be seen that the arrangement of the covering plates 2-5 is notnecessary and that these can be dispensed with if necessary.

The delay device which can contain a pyrotechnic charge, for example, isof conventional type well known to the expert so that it does not needto be shown or described here.

Several other methods of extending the fins 6-9 are naturallyconceivable within the idea of the invention. Instead of swinging out,for example, they can be extended through gaps formed in the projectile.

In its front position, the projectile comprises four nose rudders 10which can each be extended through its slot 10a, see FIG. 3, to guidethe projectile in its later, fin-stabilized part of the projectiletrajectory. The nose rudders 10 are adapted to be extended through theslots 10a when a predetermined braking of rotation is reached in theprojectile. The initiation of the extension of the nose rudders mayalternatively be effected by means of a delay device of conventionaltype, not shown. The guiding of the projectile can alternatively takeplace by means of pulses from one or more steering nozzles, in whichcase the nose rudders can be dispensed with entirely. If the projectileis provided instead with aerodynamic nose rudders, these can be extendedduring the whole trajectory time and even in the fire tube. Thispresupposes, however, that their span is less than the diameter of thebarrel. The nose rudders then are so dimenionsed that the projectile canfly spin-stabilized.

The projectile further comprises four covering plates 11-13 in a noseportion, which are adapted, through initiation by a delay device notshown, to be removed from the projectile after this has becomefin-stabilized and to expose a target tracking optical system of thelike, not shown, see FIG. 3.

Since neither the guidance system nor the target tracking optical systemconstitutes any part of the present invention, they are not shown ordescribed here in detail but the above summary description of theiroperation is regarded as sufficient.

In order to achieve fin-stabilization of the spin-stabilized projectile,according to the invention, it requires on the one hand an arrangementof the stabilizing fins 6-9 in the manner described above, and on theother hand such a dimensioning of the projectile that its aerodynamiccentre of pressure, that is to say the point where the air forces act,are situated behind the centre of gravity of the projectile in theextended position of the fins. The projectile is further so dimensionedthat its centre of pressure also lies behind the centre of gravity ofthe projectile when both the fins 6-9 and the nose rudders 10 areextended. Finally, the projectile is so dimensioned that its centre ofpressure lies somewhat in front of the centre of gravity of theprojectile when the fins 6-9 and the nose rudders 10 are in theretracted position, that is to say in the first part of the trajectoryof the projectile, when the projectile is spin-stabilized. Although forspin-stabilized projectiles in general, it applies that the centre ofpressure should lie in front of the centre of gravity in this manner, itis nevertheless conceivable, within the scope of the invention, toposition the centre of pressure in or behind the centre of gravity ofthe projectile instead.

The positioning of the centre of pressure is shown in FIG. 3 where thecentre of pressure, in the retracted position of the fins 6-9, issituated at a point C1, which lies somewhat in front of the centre ofgravity of the projectile, which is marked by G in FIG. 3. In theextended position of the fins 6-9, the centre of pressure is shiftedback to a point C2 behind the centre of gravity G. On extension of thenose rudders 10, the centre of pressure is shifted forwards somewhat toa point C3 which nevertheless also lies behind the centre of gravity G.

The mode of operation of the projectile described is as follows: Whenthe projectile is fired from a barrel not shown, it is given arelatively high speed of rotation (of the order of magnitude of 300-2000rad/sec), for example by means of conventional projectile driving bands.At a predetermined, desired point in the trajectory of the projectilethe base bleed unit 1 is thrown away so that the covering plates 2-5 areremoved and the fins 6-9 exposed. These are extended and brake therotation of the projectile. As a result of the above-mentioneddimensioning of the fins and the projectile, the projectile changes overfrom being spin-stabilized to being fin-stabilized. The terminalguidance and target-seeking function of the projectile or the triggeringof the explosive charge of the projectile with a hollow-charge effectcan now take place.

With a view to precision and range, it is usually best to extend thefins after at least half the trajectory has been covered but in certaincases an earlier extension can be advisable so as to obtain a low speedof rotation in time.

We claim:
 1. A guided projectile system for firing a projectile from abarrel and guiding the projectile through its trajectory, said guidedprojectile system comprising:rotating means for firing said projectilefrom said barrel and for rotating said projectile at a spin rate in therange of 300-2000 rad/sec to enable said projectile to be in astabilized state in its trajectory solely by rotating motion, saidprojectile travelling at least half of its trajectory outside the barrelin said stabilized state; said projectile including stabilizing finslocated in a retracted position during said stabilized state and beingextendable to an extended position at a desired point in the trajectoryof said projectile to thereby brake rotation of said projectile; saidprojectile having an aerodynamic center of pressure located in front ofthe center of gravity of said projectile when said stabilizing fins arein said retracted position, and the aerodynamic center of pressure ofsaid projectile being moved, when said stabilizing fins are in saidextended position, to be located behind the center of gravity of saidprojectile for stabilization of said projectile; and said projectile,upon extension of said stabilizing fins to said extended position forbraked rotation of said projectile, changing over from being stabilizedsolely by rotating motion to being fin-stabilized by said stabilizingfins shifting said aerodynamic center of pressure to be located behindthe center of gravity and said projectile.
 2. A guided projectile systemas claimed in claim 1, further comprising locking means for holding saidstabilizing fins in said retracted position, said locking means beingseparable from said projectile to expose said stabilizing fins at saiddesired point in the trajectory.
 3. A guided projectile system asclaimed in claim 1, wherein said stabilizing fins are formed aswrap-around fins.
 4. A guided projectile system as claimed in claim 2,wherein said stabilizing fins are formed as wrap-around fins.
 5. Aprojectile comprising:a projectile body being rotatable in the range of300-2000 rad/sec during at least half of its trajectory forstabilization of said projectile body solely by rotation; stabilizingfins located in a retracted position on said projectile body during saidat least half of the projectile body's trajectory and said stabilizingfins moving to an extended position at a desired point in the trajectoryof said projectile body to brake rotation of said projectile; and anaerodynamic center of pressure of said projectile body being located infront of the center of gravity of said projectile body during said atleast half of the projectile body's trajectory and said aerodynamiccenter of pressure being moved behind said center of gravity forstabilization of said projectile upon extension of said stabilizing finsto said extended position and for changing stabilization of saidprojectile body from stabilization solely by rotation to stabilizationby said stabilizing fins.
 6. A projectile as claimed in claim 5, furthercomprising locking means for holding said stabilizing fins in saidretracted position, said locking means being separable from saidprojectile to expose said stabilizing fins at said desired point in thetrajectory.
 7. A projectile as claimed in claim 6, wherein saidstabilizing fins are formed as wrap-around fins.
 8. A projectile asclaimed in claim 5, further comprising a base bleed unit mounted at thebase end of the projectile body.